1. Field of the Invention
The present invention relates to high power lasers, and more specifically, it relates to techniques for combining fiber lasers.
2. Description of Related Art
Fiber lasers offer many advantages over conventional lasers, including their inherent robustness, their ability to create diffraction limited beams (especially advantageous for lasers that must propagate over large distances) and their ability to efficiently radiate waste heat. The latter stems from the fact that the ratio of their surface area to their enclosed volume is relatively high. Unfortunately, the cross-sectional area of a fiber tends to be relatively small. Thus, for a given power or pulse energy, the power or energy density inside a fiber tends to be much higher than it would be inside a conventional laser. If the power or energy density becomes too large, undesirable nonlinear phenomena or catastrophic damage can be triggered, and the output of the laser can be limited.
One way to raise the nonlinear and damage thresholds is to increase the size (cross-section) of the fiber. A practical upper manufacturing bound on the fiber cross section is roughly 1000 μm2. Further scaling can make the fiber prone to bend-induced losses, and degrades the quality of the emitted beam.
A well-known alternative is to combine the outputs of many fiber lasers. Such combination techniques are generally classified as coherent or incoherent. Incoherent combination refers to the fact that the peaks and troughs of the sinusoidal electro-magnetic waves emitted by the lasers are not synchronized—that is, they vary randomly with time. A simple (though, in some cases, effective) example is to combine the lasers at various angles at a common target, similar to simultaneously shining several flashlights onto a single spot. Another incoherent technique is to combine the lasers in the spectral domain; the lasers in the array are forced to emit at a series of non-overlapping wavelengths, and their outputs are combined by a spectrally-selective device such as a diffraction grating.
Coherent combination means that, at the point of combination, the phases of the lasers are the same, or within the needs of a given application, nearly the same. While some research suggests that this co-phasing or phase-locking may occur naturally—“passive phasing”—today it is most often forced to occur by monitoring and adjusting the phases of the individual lasers with high speed electronics, feedback loops, and appropriate phase-adjusting actuators, and has been referred to as—“active phasing.”
Once the lasers in a coherent combination scheme are co-phased, either by passive or active means, they must be combined. This has been accomplished in several ways. They may be combined by a series of beam-splitters or a diffractive optical element, or they may be allowed to simply diffract and combine as they propagate toward some distant target.
Each of the various combining schemes has its own advantages and disadvantages. Actively-phased coherent beam combining has demonstrated the highest combined power of 1.2 kW, generated by an array of 1.6 lasers. This result does not compare well to the highest reported power from a single laser, though, which now stands at 6 kW. The arrayed power is relatively modest because has not been understood how to combine lasers that have broad spectral bandwidths, such as those that can now generate 6 kW. Instead, the array combines lasers having very narrow spectral widths. The narrow sources have two advantages: they allow lasers of disparate lengths to be combined—in the case of the 1.2 kW array, the lengths can differ by tens of meters—and they minimize the chromatic spread of the combined beam. The narrow sources also have a distinct disadvantage: they tend to suffer from stimulated Brillion scattering (SBS), a nonlinear phenomenon which today limits the power of the individual lasers to roughly 100 W.
Over the past several years, sizable investments and considerable effort and have been devoted to increasing the SBS power limit. One approach is to increase the size of the light-guiding portion of the fiber, but this tends to make the fibers prone to bend-induced losses and degrades the quality of the beams they emit. Other approaches are to generate a thermal gradient along the fiber's length or to create fibers that guide light but not sound. All of these approaches, combined, have raised the SBS threshold to 500 W, though it is not yet known what the limit will be for practical field deployments.
In many prior art beam-combining schemes, it is necessary to control the lengths of the constituent lasers to enhance their coupling efficiency. In such schemes, the lengths of the individual lasers are controlled to within an integer multiple of their common lasing wavelength, and in all known implementations for these schemes the multiple is quite large, ranging from 103 to 109. While such a large multiple greatly simplifies fabrication, it necessarily makes the array's temporal and spectral properties different from those of its constituent lasers. In certain applications, such as those involving the generation or amplification of mode-locked laser pulses, this can be unacceptable.